1. Technical Field of the Invention
The present invention relates to a temperature detecting device for rotating members which is, for example, used to control the temperature of a heat fixing roller in various kinds of electrophotographic image forming apparatuses.
2. Description of Related Art
In an ordinary type of a heat fixing device used in the above mentioned image forming apparatuses, a toner image electrostatically transferred to a sheet is fused and fixed by passing the sheet through the nip of a pair of fuser rollers in pressure engagement, at least one of which is heated by a heat source. The roller(s) is maintained at a temperature within a predetermined range by monitoring and controlling heat generation by the heat source based on the temperature detected by a sensor in order to conduct the above-described fixing process effectively.
FIG. 4 shows a conventional structure for supporting a sensor for controlling temperature such as a thermostat or a thermistor in a fixing device 50. A fuser roller 51 and a pressure roller 52 are mounted within the housing 60 of the fixing device 50 in tight contact with each other. Both rollers 51, 52 are heated internally by heat sources, and sensors 53, 54 are pressed against a peripheral surface of the rollers 51, 52 for temperature detection, based on which application of electricity to the heat sources is controlled so as to keep the rollers at a predetermined temperature.
A pair of plate springs 57, 58, one end of which is fixed by mounting members 55, 56 to the fixing device 50, is provided for mounting the sensors 53, 54. The other end of the plating springs 57, 58 is coupled to the sensors 53, 54. The plate springs 57, 58 are bent to form a U-shape in cross section and press the sensors 53, 54 which are coupled to the other end of the plate springs 57, 58 on to the rollers 51, 52 by their resiliency.
As described above, the sensors 53, 54 are positioned and pressed on the roller only by means of plate springs 57, 58. If the pressing force is not strong enough (if the sensors 53, 54 are not pressed tightly enough on the roller), the sensors 53, 54 may slip out of position and worst of all lose contact with the rollers 51, 52 at their either side, causing inaccuracy in temperature detection. In order to prevent an unwanted slippage of the sensors 53, 54, the pressing force by the plate springs 57, 58 is set around 100 gram-force, but yet it is not enough to keep the sensors 53, 54 stably in place and to achieve enough accuracy in detection required in recent high-speeded copying machines. On the other hand, if the pressing force is too strong, the sensors 53, 54 as well as the rollers 51, 52 will soon be worn down.